Section ‐ C
MULTIPLE ACCESS TECHNIQUE
ALLOWS INTERCONNECTION OF LARGE
NO OF EARTH STATION TERMINALS
SIMULTANEOUSLY VIA SATELLITE
MAT
• CAPACITY OF SATELLITE TRANSPONDER – 120
Mbps
• 3562 voice channels 32 Kbps
• FDM For analog
• TDM for digital
• Access scheme FDMA,TDMA,CDMA
1 Fixed
1. Fi d
2. Demand
Multiple Access
• MAT represents a traffic feature of satellite
communication.
• A technique in which several users share a
common transponder to transmit and receive
information.
• There for 3 different ways to share
transponder relative to its utility parameters
such as – bandwidth, power & time. (i.e.
FDMA, TDMA, CDMA)
FDMA
• FDMA – an access in which each user has –
9 a part of bandwidth
9
9part off power
9And all of the time
This kind of access technique used in analog
voice communication satellite.
TDMA
• TDMA – an access in which each user has –
¾ all of the power
¾ ll off the
¾All h bandwidth
b d id h
¾And part of the time
This kind of access technique is frequently used
in data and digital voice transmission of
satellite.
satellite
CDMA
• CDMA – an access in which each user has –
9 all of the bandwidth
9 allll off the
h time
i
9And only part of the power.
This kind of access technique is frequently used
in data and digital voice transmission of
satellite & analogous to TDMA
TDMA.
Important About Access Technique
• MAT has ability to support multiple
transmitters often lead to higher complexity
and cost than for a multiplexer with similar
capacity.
• Unlike multiplexers, multiple –access systems
d not usually
do ll provide
id for
f theh grouping
i off
channels into groups and super groups.
Why we need this?
• With the increase of –
9 Channel demand
9 off earth
9No h stations
i
9Efficient use of satellite transponder in
conjunction with many stations
All these factors resulted in the development
p of
multiple access techniques
MAT ‐ definition
Hence
Hence,
MULTIPLE ACCESS is a technique in which the
satellite resource (bandwidth or time) is
divided into number of non overlapping
segments and each segment is allocated
exclusively to each of the large no of earth
stations who seek to communicate with each
other.
TDMA
• Single carrier for transmission via the satellite
transponder on TDM basis
• 125μs
• Earth station has access to the entire
b d id h off the
bandwidth h transponderd
• Guard time between the individual burst
• Fixed assignment
• Demand assignment
TDMA FRAME STRUCTURE
Guard
G d
time
TRAFFIC TRAFFIC
RB1 RB2 BURST BURST RB1
1 N
TDMA FRAME
TDMA FRAME STRUCTURE
• Two reference burst
• Primary secondary reference burst
• S h i i and
Synchronization d id
identify
if the
h fframe
• Traffic burst
• Traffic station
• Guard time
• Burst time plan
REFERENCE BURST STATION
P
Preamble
bl (or
( header)
h d )
Signaling Channel or bit Control bits
Carrier and Unique Order Management Transient
clock W d
Word Wire ca Channel Timing
recovery (UW) Channel Channel
CARRIER AND CLOCK RECOVERY:
• Each burst begins,
begins
• Enables the earth station demodulator to
recover the carrier phase and regenerate the
BIT or symbol timing clock for data
demodulation
• Depends upon C/N ratio
Unique word
• Burst code word
• UW follows the CCR sequence
• marks
k eachh fframe
• Receiver frame timing that allows a station to
locate the position of traffic burst in a frame
Order wire Channel
• Voice
• Data
• Instruction
i are passed
d to and
d fro
f from
f earth
h
station
Management Channel
Reference burst is sent by reference station to
all traffic station
Instruction frame management
Burst time plan changes
Position length source or destination
Transient timing Channel
• acquisition and synchronization
• Transmit burst timing
TRAFFIC BURST STATION
P
Preamble
bl (or
( header)
h d )
Traffic data
Signaling Channel
or bit Control bits
Infor‐
Order Infor‐ Infor‐
Carrier and Unique mation
Wire Serviceca Sub mation mation
clock Word
Channel Channel burst1 Sub Sub
recovery (UW)
burst2 burstn
Unique word
• Marks time of occurrence of the traffic burst
• Provides receiver burst timing
• Extract only
l wantedd sub
b burst
b within
i hi the
h
traffic burst
Service Channel
Traffic station status to the Reference Station
TDMA frame efficiency
η = 1 −
∑ t
T F
( n + 2 ) P
∑ t =
R
R satellite transmission link bit rate
N no of traffic burst in a frame
P no of bits in the preamble plus guard time
TF is the frame period
R
ν =
rη
No of voice channels can be calculated
